HELP!! PLEASE!!

Guys!
Please help, my plants are 12 weeks old now, veg for 6, flowered for 6.
My plants looked really down and the lower leaves where turning yellow and starting to fall off, a little too soon for my liking seeing as theres another 6 weeks to go!
:confused:
Not much was happening and I figured that they must need some fert or something.
I added some "Miracle Gro pour & feed" added less than the recommended dosage hoping to see some good results.
I've noticed now that the yellowing has increased and is even starting to affect new plant life at the lower of my plants!:( :(
My tips are curled too, I heard that was to do with the PH or calcium?
Basically, my plants arent doing too great, although some appear fine and unaffected.
It's weird, what am I doing wrong:mad:
PLEASEEEEEE Help:D

what way is your leaf tips curled up or down?
also what size pots are you using, what type of lighting are you using?

The tips are curling up, the pots are 7 litre or 1 1/2 gallon pots.
I'm using a 400w HPS bulb, and its about 1foot from the tip of the highest plant.

what r your temps like?

yellowing leaves are ok, some plants change and lose them all in a week, others lose leaves for several weeks (like my sage) no worries there, but as for the tip curl, what's the npk value of your ferts. did you get the kind with NO trace elements?

what is the ph of your soil?

would try using epsom salt to eliminate the possibility of mg defficiency it can cause yellowing and upward leaf curl just mix a teaspoon in a gallon of water and feed the plants with it.
ph is important and if you have no idea where its at it could be causing your problems some nutrients may be available but if the ph is out of range they will not be used by the plant, in soil you want it slightly acidic around 6.5.
your pots are a little small also for a 3 month old plant you should consider 3 gallon pots or even larger.
yellowing leaves is not normal, it is a common mistake to think that yellowing is normal during flowering but with the right nutrients on an indoor grow where temps dont really come into play, the plant should still be very healthy green even at harvest time if its yellowing your not giving it the nutrients it needs so its forced to pull them from the leaves instead which can hurt yield.

Hi guys, thanks for the help, added some epsom salts and things started to look up, had some colour returning to the plants, and i was feeling good.
Unfortunately, I came back today, and the plants are once again yellowing.
I haven't added any extra nutes and I didnt flush.
So the nutes in the soil should still be there, unless your telling me my plants eaten a months food in 2 weeks!?!?
I'm almost on the home straight, this weekend will mean I have 3 weeks until it should be at its peak.
My question is:
Should I flush them or not?
If i do flush them, and it is a def of some sort, then the yellowing will increase
If i don't, and its nute burn, they will die.
New life is once again going yellow.
Any thoughts or views would be appreciated, (Ps I haven't moved them)